My Math Forum Prove the difference between numbers

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 March 27th, 2012, 05:46 PM #1 Newbie   Joined: Jan 2012 Posts: 28 Thanks: 0 Prove the difference between numbers The sum of the digits of a two-digit prime number is subtracted from the number. Prove that the difference cannot be a prime number.
 March 27th, 2012, 06:47 PM #2 Senior Member     Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 521 Math Focus: Calculus/ODEs Re: Prove the difference between numbers Let A be the tens place digit and B be the units digits, the the difference is: $$$10A+B$$-$$A+B$$=9A$ Since 9 is composite, so must 9A be composite.

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